A wide variety of off-line LED drivers are known. For example, a capacitive drop off-line LED driver from On Semiconductor (Application Note AND8146/D) is a non-isolated driver with low efficiency, is limited to deliver relatively low power, and at most can deliver a constant current to the LED with no temperature compensation, no dimming arrangements, and no voltage or current protection for the LED.
Isolated off-line LED drivers also have wide-ranging components and/or characteristics, such as a line frequency transformer and current regulator (On Semiconductor Application Note AND 8137/D); a current mode controller (On Semiconductor Application Note AND8136/D: a white LED luminary light control system (U.S. Pat. No. 6,441,558); LED driving circuitry with light intensity feedback to control output light intensity of an LED (U.S. Pat. No. 6,153,985); a non-linear light-emitting load current control (U.S. Pat. No. 6,400,102); a flyback as an LED Driver (U.S. Pat. No. 6,304,464); a power supply for an LED (U.S. Pat. No. 6,557,512); a voltage booster for enabling the power factor controller of a LED lamp upon a low AC or DC supply (U.S. Pat. No. 6,091,614); and an inductor based boost converter (e.g., LT 1932 from Linear Technology or NTC5006 from On—Semiconductor).
In general, these various LED drivers are overly complicated, such as using secondary side signals (feedback loops) which have to be coupled with the controller primary side, across the isolation provided by one or more transformers. Many utilize a current mode regulator with a ramp compensation of a pulse width modulation (“PWM”) circuit. Such current mode regulators require relatively many functional circuits while nonetheless continuing to exhibit stability problems when used in the continuous current mode with a duty cycle (or duty ratio) over fifty percent. Various prior art attempts to solve these problems utilized a constant off time boost converter or hysteric pulse train booster. While these prior art solutions addressed problems of instability, these hysteretic pulse train converters exhibit other difficulties, such as electromagnetic interference, inability to meet other electromagnetic compatibility requirements, and are comparatively inefficient. Other approaches, such as in U.S. Pat. No. 6,515,434 B1 and U.S. Pat. No. 747,420, provide solutions outside the original power converter stages, adding additional feedback and other circuits, which render the LED driver even larger and more complicated.
Many of these power supplies (i.e., drivers) are effectively incompatible with the existing lighting system infrastructure, such as the lighting systems typically utilized for incandescent or fluorescent lighting. These power supplies, generally implemented as a form of switching power supplies, are particularly incompatible with phase-modulating “dimmer” switches utilized to alter the brightness or intensity of light output from incandescent bulbs. Incandescent lamps primarily utilize phase modulation for dimming brightness or intensity, through triac switches, to control the power sent to the bulb. Accordingly, replacement of incandescent lamps by LEDs is facing a challenge: either do a complete rewiring of the lighting infrastructure, which is expensive and unlikely to occur, or develop new LED drivers compatible with phase modulation of the input AC voltage by commercially available and already installed dimmers switches. In addition, as many incandescent lamps will likely remain in any given lighting environment, it would be highly desirable to enable LEDs and incandescent lamps to operate in parallel and under common control.
One prior art approach to this problem is found in Elliot, US Patent Application Publication No. 2005/0168168, entitled “Light Dimmer for LED and Incandescent lamps”, in which incandescent lamps and LEDs are connected to a common lamp power bus, with the light output intensity controlled using a composite waveform, having two power components. This proposal is complicated, requires excessively many components to implement, and is not particularly oriented to AC utility lighting.
Another prior art approach is described in Mednik et al., U.S. Pat. No. 6,781,351, entitled “AC/DC Cascaded Power Converters having High DC Conversion Ratio and Improved AC Line Harmonics”, and in publication Alex Mednik “Switch Mode Technique in Driving HB LEDs”, Proceedings of the Conference Light Emitting Diodes, 2005, which disclose an off-line LED driver with a power factor correction capability. When coupled with a dimmer, however, its LED regulation is poor and it does not completely support stable operation of the dimmer in the full range of output loads, specifically when both incandescent and LED lamps are being used in parallel.
FIG. 1 is a circuit diagram of a prior art current regulator 50 connected to a dimmer switch 75 which provides phase modulation. FIG. 2 is a circuit diagram of such a prior art dimmer switch 75. The time constant of resistor 76 (R1) and capacitor 77 (C1) control the firing angle “α” of the triac 80 (illustrated in FIG. 4). The diac 85 is used to maximize symmetry between the firing angle for the positive and negative half cycles of the input AC line voltage (35). Capacitor 45 (C2) and inductor 40 (L1) form a low pass filter to help reduce noise, generated by the dimmer switch 75. A triac 80 is a switching device effectively equivalent to reverse parallel Silicon Controlled Rectifiers (SCR), sharing a common gate. The single SCR is a gate controlled semiconductor that behaves like a diode when turned on. The gate (70) signal is used to turn the device on and the load current is used to hold the device on. Thus, the gate signal cannot turn the SCR off and will remain on until the load current goes to zero. A triac behaves like a SCR but conducts in both directions. Triacs are well known to have different turn on thresholds for positive and negative conduction. This difference is usually minimized by using a diac 85 coupled to the triac gate 70 to control the turn on voltage of the triac 80.
Triacs 80 also have minimum latching and holding currents. The latching current is the minimum current required to turn on the triac 80 when given a sufficient gate pulse. The holding current is the minimum current required to hold the triac 80 in an on state once conducting. When the current drops below this holding current, the triac 80 will turn off. The latching current is typically higher than the holding current. For dimmer switches that use triacs, capable of switching 3 to 8 A, the holding and latching currents are on the order of 10 mA to about 70 mA.
The firing angle (α) of the triac 80 controls the delay from the zero crossing of the AC line, and is limited between 0° and 180°, with 0° equating to full power and 180° to no power delivered to the load, with an exemplary phase-modulated output voltage illustrated in FIG. 4. A typical dimmer switch, for example, may have minimum and maximum α values of about 25° and 155° respectively, allowing about 98% to 2% of power to flow to the load compared to operation directly from the AC mains (AC line voltage (35)). Referring to FIG. 2, the firing angle is determined by the RC time constant of capacitor 77 (C1), resistor 76 (R1), and the impedance of the load, such as an incandescent bulb or an LED driver circuit (ZLOAD). In typical dimming applications, ZLOAD will be orders of magnitude lower than R1 and resistive, thus will not affect the firing angle appreciably. When the load is comparable to R1 or is not resistive, however, the firing angle and behavior of the dimmer switch can change dramatically.
Typical prior art, off-line AC/DC converters that drive LEDs using phase modulation from a dimmer switch have several problems associated with providing a quality drive to LEDs, such as: (1) such phase modulation from a dimmer switch can produce a low frequency (about 120 Hz) in the optical output, which can be detected by a human eye or otherwise create a reaction in people to the oscillating light; (2) filtering the input voltage may require quite a substantial value of the input capacitor, compromising both the size of the converter and its life; (3) when the triac 80 is turned on, a large inrush current may be created, due to a low impedance of the input filter, which may damage elements of both the dimmer switch 75 and any LED driver; and (4) power management controllers are typically not designed to operate in an environment having phase modulation of input voltage and could malfunction.
For example, as illustrated in FIG. 5, a switching off-line LED driver 90 typically includes a full wave rectifier 20 with a capacitive filter 15, which allows current to flow to the filter capacitor (CFILT) 15, when the input voltage is greater than the voltage across the capacitor. The inrush current to the capacitor is limited by the resistance in series with the capacitor. Under normal operating conditions there may be a Negative Temperature Coefficient resistor (NTC) or thermistor in series with the capacitor to minimize inrush current during initial charging. This resistance will be significantly reduced during operation, allowing for fast capacitor charging. This circuit will continuously peak charge the capacitor to the peak voltage of the input waveform, 169 V DC for standard 120 V AC line voltage.
When used with a dimmer switch 75, however, the charging current of the filter capacitor is limited by the dimming resistance R1 (of resistor 76) and is ICHARGE=(VIN−VLOAD−VC1)/R1 (FIGS. 2 and 5). The voltage across the filter capacitor can be approximated to a DC voltage source due to the large difference between C1 (77) and CFILT (15). The charging current of the filter capacitor is also the charging current for C1, which controls the firing angle of the dimmer. The charging current for C1 will be decreased from normal dimmer operation due to the large voltage drop across the filter capacitor 15. For large values of VC1, the current into C1 will be small and thus slowly charge. As a consequence, the small charging current may not be enough to charge C1 to the diac 85 breakover voltage during one half cycle. If the breakover voltage is not reached, the triac 80 will not turn on. This will continue through many cycles until the voltage on the filter capacitor is small enough to allow C1 to charge to the breakover voltage. Once the breakover voltage has been reached, the triac 80 will turn on and the capacitor will charge to the peak value of the remaining half cycle input voltage.
When a dimmer switch is used with a load drawing or sinking a small amount of current, ILOAD<holding current for all values of the AC input, the triac 80 will provide inconsistent behavior unsuitable for applications with LED drivers. The nominal firing angle will increase due to the increased resistance of ZLOAD 81. When the capacitor (C1) voltage exceeds the diac breakover voltage, the diac 85 will discharge the capacitor into the gate of the triac 80, momentarily turning the triac on. Because the load resistance is too high to allow the necessary holding current, however, the triac 80 will then turn off. When the triac turns off, the capacitor C1 begins charging again through R1 and ZLOAD (81). If there is enough time remaining in the half cycle, the triac will fire again, and this process repeats itself through each half cycle. Such premature and unsustainable on-states of a triac 80 are illustrated in FIG. 3, showing the premature startup attempts of the triac 80 which can cause perceptible LED flicker.
Accordingly, a need remains for an LED driver circuit which can operate consistently with a typical or standard dimmer switch of the existing lighting infrastructure and avoid the problems discussed above, while providing the environmental and energy-saving benefits of LED lighting. Such an LED driver circuit should be able to be controlled by standard switches of the existing lighting infrastructure to provide the same regulated brightness, such as for productivity, flexibility, aesthetics, ambience, and energy savings. Such an LED driver circuit should be able to operate not only alone, but also in parallel with other types of lighting, such as incandescent lighting, and be controllable by the same switches, such as dimmer switches or other adaptive or programmable switches used with such incandescent lighting. Such an LED driver circuit should also be operable within the existing lighting infrastructure, without the need for re-wiring or other retrofitting.